In 1969/70 models, the arrangement of the steering head races is fairly
obvious, however the problems start occurring with the introduction of the
forks used with the conical hub brake in 1971. Although the yokes used in
these forks are similar to the Triumph/BSA Twin models that used them, there
are important differences with regard to the location of the steering races,
and the length of the steering stem. The bottom yoke is the only one
affected, and it is very dangerous to try to use a twin yoke on a triple.
The frame headstock lug used in all T150 Trident models was the same general
forging as that used in the 1963-1970 6T/TR6/T120 Triumph twin models, the
later Daytona models, and generally dimensionally the same as BSA Rocket 3, and
therefore the headrace cops and cones were also (nearly) identical. However,
the fork yokes used for the 1971/72 Triple drum brake and 1973/75 Triple disc
brake models used a smaller diameter stem than the 1969/70 Triple models, or
the other earlier twins and another piece, part number 97-4145 was fitted into
the arrangement to adapt the smaller stem to the existing top cone. The dust
shield fitted in the top cone remained unchanged. As this adaptor is a
separate piece it is not supplied in headrace "sets" and has to be moved
from the existing top cone to the replacement. The bottom cup and cone
remained unchanged, with in Triple yokes, the smaller diameter stem having a
thickened lower register to locate the lower cone by its inside diameter.
The small stem design, when used in the "oil-in-frame" twin was engineered
to allow the use of two taper roller headraces in a smaller headstock. In
this version of the arrangement the taper roller sat on the thickened lower
area, around a ground raised area of stem. The Trident did not receive this
modification until the T160 model of 1975, which also had a longer stem too,
so the bottom yoke is specific to that model.The very few disc braked
Daytona models produced used the old headstock arrangements, and therefore
the fork yokes from the T150, and the adaptor.
This knowledge usefully leads to the choice of the correct method of fitting
a set of T120 or T140 disc brake forks and wheel to an earlier T100/T120
Twin or T150/A75 Triple. The "oil in frame" twin that carried the disc brake
fork used the small diameter steering stem and taper roller headraces. The
bottom yoke is not suitable for fitting these forks into an earlier twin or
triple because the location arrangements for the cup and cone headraces are
incorrectly sized or positioned. The correct bottom yoke to use for this
modification is the one from a 1973/75 T150 Trident. Either style of disc
fork top yoke, with the instruments mounted under the dummy fork top-nuts,
or in the T160 style pods, may be used when doing this conversion. The
97-4145 adaptor must, of course, be fitted. There are also damping and
springing specification changes between twin and triple, but these internals
are interchangeable. The overall length of the steering stem was also
different between the variations in yokes, and only the T150 disc type yoke
has the correct length for this "Retro-fitting".
The caged ball races used in later original production are now not available, and each cup/cone assembly must be fitted with 20 1/4" diameter balls. I do wish that the original pieces were still available, because assembly was so much easier! There are now a number of folk offering taper roller bearing conversions to replace these cup/cones. If one of these is used then it is worthwhile carefully inspecting the arrangement before fitting to ensure that it will be possible to easily remove the outer race from the frame at a later date. It is also worth noting that the component parts from one manufacturers taper roller kit may not fit with components from another manufacturer.
A little caution is required when working on MK2 A75s as this model used a more shallow top dust cover in its top cone. These are, I believe, not generally available and the Triumph item may have to be shortened to allow for frame clearance. I also believe the same is true for the slightly different X-75 arrangements too.
Inspection of a number of bikes over the years also shows that some people
are also making an error with the assembly of the yokes with regard to the
headlamp supports. The top yoke of our machines does not sit against the
underside of the central bearing adjusting nut. The top yoke should sit
down on to the dust excluder on 1969/70 models, and the 97-4145 adaptor on
disc brake and conical models. On the early and late machines this will also
have the correct effect of locating the headlamp shrouds properly into the
recess in the underside of the top yoke, and on conical models does not
strain the wire stays. It is the job of the stanchion pinch bolt in the
yoke(s) to keep the yokes in the correct position, not the underside of the
hexagon of the centre nut.
Copyright © 2003 Philip Pick
Version 0.1 Date June 2003